“One swallow does not a summer make” is an old proverb. “One game/loss/win doesn’t define a season” is a sports proverb.
   
“I also agree that one ugly loss doesn’t define a season” was cited in a newsgroup in 1995. “One game doesn’t define a season” was cited in a 1999 newspaper. “One inning doesn’t define a season” was cited in 2003.
 
There are exceptions to the sports proverb. A football bowl game loss (such as a Super Bowl loss) doesn’t define a season as a loss, but it does define a season (somewhat against the proverb). The saying is often reserved for regular season games.
 
       
Wiktionary: one swallow does not a summer make
Etymology
An allusion to the return of migrating swallows at the start of the summer season. From a remark by Aristotle (384 BCE - 322 BCE): “One swallow does not a summer make, nor one fine day; similarly one day or brief time of happiness does not make a person entirely happy.”
Proverb
one swallow does not a summer make
1. One instance of an event (such as the arrival of a single bird) does not necessarily indicate a trend.
   
Google Groups: alt.sports.football.pro.miami-dolphins
Fairweather Fins Fans
Rick Hicks
10/18/95
(...)
I also agree that one ugly loss doesn’t define a season.
 
22 January 1999, Cincinnati (OH) Enquirer, “Bearcats make a loud statement,” pg. B1:
One game doesn’t define a season. But it helps. The 81-55 statement the Bearcats laid on the Cardinals had some meanings all its own.
 
28 May 2003, Star-Gazette (Elmira, NY), “CV wins in district softball,” pg. B1:
“It’s unfortunate that we lost our composure in the last inning, but one inning doesn’t define a season,” Edison coach Mick Cornacchio said. “The girls played hard all season.”
   
3 November 2003, Asbury (NJ) Park Press, “Monmouth ready to move on” by Tony Graham, pg. 9:
IT’S not exactly back to the drawing board for the Monmouth University football team.
 
“One game doesn’t define a season,” said Monmouth coach Kevin Callahan. “Once we put this one to rest we’ll be ready to move on and be ready to go.”
 
Google News Archive
14 September 2004, Fort Scott (KS) Tribune, “Panthers fall to Packers, lose SMith to broken leg” by Jenna Fryer (AP), pg. 7, col. 3:
“One game doesn’t define a season, and I really think we’re better than we showed,” center Jeff Mitchell said.
 
Twitter
David Kim
‏@Davidkim24
@SportsNation stick with texans. one week doesn’t define a season.
4:09 PM - 14 Sep 2009
 
Twitter
Cleveland Cavaliers
‏@cavs
One game doesn’t define a season. See what The Optimist has to say as the Cavs take on the Raptors tonight at 7:00 pm. http://bit.ly/1Dxa2J
1:14 PM - 28 Oct 2009
 
Twitter
NOT the real Maurer
‏@Drew_Maurer
@Sandles_Nation @rlenhard15 when they say one play doesn’t define a season, they were wrong. It’ll be in infamy forever but no, I’m not mad
7:21 PM - 1 Jan 2016
 
Observer-Reporter (Washington, PA)
WPIAL releases pairings for baseball playoffs
By Lance Lysowski May 13, 2016
(...)
The old adage goes that one game doesn’t define a season, but one game certainly set off a chain of events that has Peters Township seeded lower than the Big Macs and one win away from having to face the top team in the state.